Method of cleaning porous body, and process for producing porous body, non-porous film or bonded substrate

ABSTRACT

In order to clean a porous body in a short time without causing any change in its structure, in a cleaning method of cleaning a porous body formed by anodization, the porous body is cleaned after the anodization is completed, with a cleaning solution containing at least one of an alcohol and acetic acid.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method of cleaning a porous body, anda process for producing a porous body, a non-porous film or a bondedsubstrate. More particularly, this invention belongs to a technicalfield of a production process in which a method of cleaning a porousbody after anodization can be improved to form a non-porous film havinga uniform thickness.

[0003] 2. Related Background Art

[0004] In the following description, the case of porous silicon is takenas an example of the porous body.

[0005] Porous silicon was discovered by A. Uhlir and D. R. Turner in thecourse of their researches made on the electrolytic polishing ofsingle-crystal silicon biased to positive potential, in an aqueoushydrogen fluoride (hereinafter often simply “HF”) solution (i.e.,hydrofluoric acid).

[0006] Since then, with utilization of the property of porous siliconthat is rich in reactivity, studies are made on its application to thestep of interelement isolation that requires formation of a thickinsulating material in a silicon integrated-circuit fabrication process,and a technique of FIPOS (full isolation by porous oxidized silicon) hasbeen developed in which device elements are full-isolated by a poroussilicon oxide film (K. Imai, Solid-state Electron 24, 159, 1981).

[0007] Recently, also proposed in, e.g., Japanese Patent No. 2608351 andU.S. Pat. No. 5,371,037, is a technique in which a silicon epitaxiallayer grown on a porous silicon substrate is bonded to the surface of anamorphous substrate or single-crystal silicon substrate optionally viaan oxide film to obtain an SOI (silicon on insulator) substrate.

[0008] Besides, Japanese Patent Application Laid-Open No. 6-338631discloses a technique taking note of porous silicon as a light-emittingmaterial such as what is called a photoluminescence material orelectroluminescence material.

[0009] Anodization is commonly used to form porous bodies.

[0010] As an example of the formation of porous bodies, an apparatus forproducing porous silicon by subjecting a silicon substrate toanodization is shown in FIG. 18.

[0011] The apparatus or unit shown in FIG. 18 is one disclosed inJapanese Patent Application Laid-Open No. 60-94737. This anodizationapparatus comprises anodization baths 61 and 62 made of an HF-resistantmaterial TEFLON (a trademark of Du Pont, U.S.A.) which are so providedas to hold between them a silicon substrate W as a treatment target. Thebaths 61 and 62 are provided with a negative electrode 63 and a positiveelectrode 64, respectively. The baths 61 and 62 have grooves made in thesidewalls coming into contact with the silicon substrate W. In thesegrooves, sealing members such as O-rings 65 and 66 made of fluorinerubber are respectively fitted. Thus, the baths 61 and 62 holding thesilicon substrate W are sealed with the O-rings 65 and 66. The baths 61and 62 sealed in this way are filled with aqueous HF solutions 67 and68, respectively.

[0012] Some anodization apparatus are also proposed besides this.

[0013] Meanwhile, with regard to methods of cleaning poroussemiconductor substrates after the anodization has been effected, anexample is reported in Japanese Patent Application Laid-Open No.10-64870, but it appears that very few examples have been reported otherthan this.

[0014] For the cleaning of porous bodies structurally having a highsurface activity, liquid chemicals such as an aqueous solution ofsulfuric acid and hydrogen peroxide (hereinafter “SPM”), an aqueoussolution of ammonia and hydrogen peroxide (hereinafter “SC-1”) and anaqueous solution of hydrochloric acid and hydrogen peroxide (hereinafter“SC-2”) can not be used, which are commonly used to remove organicmatters, particle deposits or metal deposits. Accordingly, a cleaningmethod is proposed in which pure water provided with ultrasonic energyis used in place of these to remove foreign matters having adhered toporous layer surfaces, as disclosed in Japanese Patent ApplicationLaid-Open No. 10-64870. FIG. 19 is a flow chart of the steps of suchcleaning. A porous body having come out after it has been anodized in astep STP1, is cleaned in a step STP2 with the pure water provided withultrasonic energy, followed by drying in a step STP3.

[0015] The above publication also discloses a method in which the porouslayer surface is hydrophilic-treated with ozone water or hydrogenperoxide water and thereafter cleaned with the pure water provided withultrasonic energy.

[0016] However, in the cleaning of porous semiconductors, of course thecleaning of surfaces is indispensable, but it is important how theanodizing electrolytic solution having entered fine pores are removed.This is because, however the surface has been cleaned, the electrolyticsolution (usually an aqueous HF solution with a concentration of 10% byweight to 50% by weight) remaining in pores causes a change in structureof the porous body.

[0017] In addition, the HF having vaporized gradually as HF gas from theinterior of pores may corrode the surrounding devices. Moreover,particles generated as a result of corrosion may contaminate thesubstrate.

[0018] Furthermore, since it takes a time to replace the HF in poreswith the pure water, the cleaning with pure water must be carried outfor a long time. In such a case, the porous body may crush in the purewater to cause a difficulty that particles are generated in a largequantity.

[0019] Such porous bodies are also preferably used in the production ofbonded substrates utilized in SOI techniques.

[0020]FIG. 20 is a diagrammatic view to illustrate a process ofproducing a bonded substrate.

[0021] First, in a step S1, a non-porous substrate 1 such as asingle-crystal silicon wafer is prepared and its surface is made porousby anodization to form a porous layer 2 formed of single-crystalsilicon.

[0022] Next, in a step S2, the porous layer 2 is cleaned with pure waterto wash away the foreign matters adhering to the porous layer or theelectrolytic solution for anodization.

[0023] Subsequently, in a step S3, a non-porous layer 3 formed ofsingle-crystal silicon is epitaxially grown on the porous layer 2 by,e.g., CVD (chemical vapor deposition).

[0024] Then, in a step S4, the surface of the non-porous layer isthermally oxidized to form an insulating layer 4.

[0025] In a subsequent step S5, the surface of the insulating layer 4 isbonded to a supporting base 5 prepared separately, to from a multi-layerstructure in which the non-porous layer 4 is positioned inside.

[0026] In a step S6, the non-porous portion of the substrate 1, havingremained without being made porous, is removed by grinding and by ionetching subsequent thereto.

[0027] Then, in a step S7, the porous layer 2 thus uncovered is removedby etching with an aqueous solution containing HF and H₂O₂.

[0028] The surface of the non-porous semiconductor layer may optionallybe smoothed by heat treatment made in a reducing atmosphere containinghydrogen, thus a bonded substrate is obtained which has a thinsemiconductor layer on an insulating layer formed on a supporting base.FIG. 21 illustrates the top surface of a bonded substrate obtained inthis way. Reference numeral 12 denotes a notch.

[0029] However, upon observation of the surface of the non-poroussemiconductor layer thus formed, few circular spots 11 (hazes) whichlook optically different from their surrounding area are often seen. Asa result of careful observation of the circular spots 11, this has beenfound due to the fact that the non-porous layer present on theinsulating layer formed on the supporting base stands locally small inthickness (or thin). Namely, the non-porous layer proved to have locallycaused microscopically uneven film thickness.

SUMMARY OF THE INVENTION

[0030] An object of the present invention is to provide a porous-bodycleaning method by which the anodizing solution can well be removed fromthe porous body without causing any change in porous structure of theporous body and even when cleaned for a short time, and provide aprocess for producing a porous body.

[0031] Another object of the present invention is to provide aporous-body cleaning method which may hardly cause corrosion of thesurrounding devices, and a process for producing a porous body.

[0032] The cleaning method of the present invention is a cleaning methodof cleaning a porous body formed by anodization, and is characterized bycomprising the step of cleaning the porous body after the anodization iscompleted, with a cleaning solution containing at least one of analcohol and acetic acid.

[0033] The porous-body production process of the present invention ischaracterized by comprising the step of subjecting a non-porous body toanodization and thereafter cleaning a porous body with a cleaningsolution containing at least one of an alcohol and acetic acid.

[0034] Still another object of the present invention is to provide aprocess for producing a non-porous film and a bonded substrate which arefree of uneven film thickness.

[0035] The above process of the present invention is characterized by aprocess for producing a non-porous film or a bonded substrate, theprocess comprising the steps of forming a porous layer by anodization,bonding to a supporting base a non-porous layer formed on the porouslayer, and removing the porous layer, wherein;

[0036] the process further comprises the step of, after the anodizationis completed, cleaning the porous layer with a cleaning solutioncontaining at least one of an alcohol and acetic acid.

[0037] At first, the present inventors had thought that the hazes statedpreviously occur because of conditions at the time of anodization oretching conditions which are not optimized when the porous layer isremoved from the surface of the non-porous layer. However, anyadjustment of these conditions was found not to so much affect thecontrolling of uneven film thickness. Then, further studies made by thepresent inventors have revealed that the uneven film thickness is causeddepending on the treatment made after anodization.

[0038] Accordingly, in the present invention, the above cleaning methodis employed in wet cleaning after anodization so that the non-porouslayer of the bonded substrate may hardly cause uneven film thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039]FIG. 1 is a flow chart of the steps of cleaning a porous bodyaccording to the present invention.

[0040]FIG. 2 is a diagrammatic cross-sectional view of a porous bodystanding immediately after anodization.

[0041]FIG. 3 is a diagrammatic cross-sectional view of a porous bodyafter the porous body standing as shown in FIG. 2 has been cleaned withpure water immediately after anodization.

[0042]FIG. 4 is a diagrammatic view to illustrate the action of thecleaning with an alcohol according to the present invention.

[0043]FIG. 5 is a diagrammatic view to illustrate the action of thecleaning with pure water, used in the present invention.

[0044]FIG. 6 is a diagrammatic view showing an anodization-and-cleaningunit used in the present invention.

[0045]FIG. 7 is a diagrammatic view showing a system having ananodization unit and a cleaning unit separately, used in the presentinvention as another type.

[0046]FIG. 8 is a diagrammatic view showing a cleaning-and-drying unitused in the present invention as another type.

[0047]FIG. 9 is a diagrammatic view showing a cleaning-and-drying unitused in the present invention as still another type.

[0048]FIG. 10 is a flow chart of a process for producing a bondedsubstrate according to the present invention.

[0049]FIG. 11 is a diagrammatic view showing part of an anodization unitused in the present invention.

[0050]FIG. 12 is a diagrammatic view showing an anodization unit used inthe present invention.

[0051]FIG. 13 is a diagrammatic view showing a system having ananodization unit and a cleaning unit separately, used in the presentinvention.

[0052]FIG. 14 is a diagrammatic view showing a system having ananodization unit and a cleaning unit separately, used in the presentinvention as another type.

[0053]FIG. 15A, FIG. 15B, FIG. 15C and FIG. 15D are diagrammatic viewsto illustrate the action of cleaning by a conventional method.

[0054]FIG. 16A, FIG. 16B and FIG. 16C are diagrammatic views toillustrate the action of cleaning according to the present invention.

[0055]FIG. 17 is a top plan view showing an appearance of a bondedsubstrate obtained by the process of the present invention.

[0056]FIG. 18 is a diagrammatic view showing a conventional anodizationunit.

[0057]FIG. 19 is a flow chart of the steps of cleaning a porous body ina conventional method.

[0058]FIG. 20 is a flow chart of a conventional process for producing abonded substrate.

[0059]FIG. 21 is a top plan view showing an appearance of a bondedsubstrate obtained by a conventional process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0060]FIG. 1 is a flow chart of the cleaning method of the presentinvention.

[0061] First, in a step S1, a non-porous body is treated by anodizationto form a porous body. (“Porous body” is herein meant to embrace amaterial which is porous on the whole and a material having a porouslayer described later.)

[0062] Subsequently, in a step S2, the porous body is cleaned byimmersing it in a 100% alcohol or an aqueous alcohol solution, by addinga 100% alcohol or an aqueous alcohol solution dropwise to the porousbody, or by exposing the porous body to the vapor of a 100% alcohol oran aqueous alcohol solution.

[0063] Next, optionally in a step S3, the porous body is cleaned withpure water. In this step, too, the porous body may be immersed in thepure water, the pure water may be added dropwise to the porous body, orthe porous body may be exposed to water vapor (steam). Here, it is alsopreferable to clean it with pure water provided with ultrasonic energyby the use of an ultrasonic vibrator.

[0064] Then, in a step S4, the porous body is dried to complete a seriesof cleaning steps.

[0065] Usually, in the case where semiconductors such as silicon aresubjected to anodization to form porous bodies, the anodization iseffected by applying an electric field to the silicon substrate in anaqueous HF solution having relatively a high concentration. The siliconsurface opposing the negative electrode (this silicon surface servessubstantially as the anode) is so etched that fine pores extending alongthe direction of the electric field are formed, so that it comes to havea porous structure.

[0066] The pores thus formed have a size distributing from tens ofnanometers to hundreds of nanometers, and have a density reaching 10¹¹pores/cm² or higher. The size and density of the pores change dependingon the conditions of anodization, i.e., HF concentration, values ofanodization voltage or anodization electric current, and conductivitytype or specific resistance of the substrate. Their porosity may becontrolled by adjusting these conditions, whereby structures most suitedto light-emitting materials or porous bodies most suited to basestructures for epitaxial growth can be produced relatively with ease.

[0067] However, even porous bodies obtained exactly as designed, theymay cause a structural change of porous layers or may be involved insecondary contamination, unless they are sufficiently cleaned afteranodization. FIGS. 2 to 5 are diagrammatic views to illustrate how theinterior of pores of the porous body stands.

[0068] Such structural change and secondary contamination are caused by,as shown in FIG. 2, aqueous HF solution 80 remaining in the pores asstated previously and evaporating gradually in the form of gas.Accordingly, a cleaning method is necessary that does not allow anyanodizing solution aqueous HF solution 80 to remain in the pores.Reference numeral 81 denotes pore walls of the porous body.

[0069] If the substrate is only cleaned with pure water after theanodization is completed, although the HF component on the layer surfaceof the porous layer may be removable, it is difficult to remove the HFremaining in the pores. This is because, as shown in FIG. 3, the layersurface of the porous layer turns hydrophobic as a result of its contactwith the aqueous HF solution 82 and, even though it is washed with waterthereafter, it has become difficult for the water to penetrate into thepores. Stated specifically, in measurement of changes with time ofspecific resistance of the pure water in which the porous body havingbeen subjected to anodization stand immersed, there occurs a phenomenonthat the value of specific resistance does not return to the originalvalue. This is just because the HF having gradually vaporized in thepores dissolves in the pure water at a low rate.

[0070] In order to clean the interior of pores, it is alsoexperimentally necessary at least to replace the anodizing solution inthe bath with the pure water within 3 minutes after the anodization iscompleted, and to carry out rinsing with pure water sufficiently. Suchprocedure, however, narrows the freedom of the cleaning process and alsomakes the designing of cleaning unit restrictive.

[0071] The present inventors repeated various experiments. As a result,they have discovered that the cleaning with pure water to which a 100%alcohol or an aqueous alcohol solution has been added readily enablesreplacement of the anodizing solution in pores with the cleaningsolution, and thus has accomplished the present invention.

[0072] Once the porous body is immersed in the cleaning solutioncontaining an alcohol, as shown in FIG. 4 the cleaning solution 83penetrates into fine pores to become intermingled with the anodizingsolution having remained in the pores. Then, even when the porous bodyis exposed to the atmosphere as it is, its surface is kept standing wetfor a while. More specifically, the surface of the porous body, too, iscovered with a layer 84 formed of the cleaning solution or anodizingsolution. This porous body is anew immersed in pure water 85 as shown inFIG. 5, so that the water 85 penetrates into the pores with ease, thusthe anodizing solution 80 remaining therein can be replaced with thepure water 85.

[0073] After that, even if the pure water 85 has remained in the poresof the porous body, the water evaporates naturally. In that course, whatevaporates is only the water, and hence it little causes the corrosionof surrounding devices or the change of properties or deterioration ofthe porous body's structure itself.

[0074] The porous body used in the present invention may includesemiconductors such as Si, Ge, GaAs, GaAlAs, SiC, SiGe and C. Inparticular, porous silicon having a porosity lower than 70% is preferredas a base material for epitaxial growth, and silicon having a porosityof 70% or higher as a light-emitting material.

[0075] The anodization used in the present invention is carried out inan aqueous HF solution, or an aqueous solution of HF and an alcohol.

[0076] As the alcohol used in the cleaning step of the presentinvention, usable are methanol, ethanol and propanol. The cleaningsolution may be any those containing at least 4% by weight, andpreferably at least 10% by weight, of the alcohol.

[0077] After the cleaning with the cleaning solution containing analcohol, the porous body may preferably be cleaned (or rinsed) with purewater. Here, as mentioned previously it may be cleaned with pure waterprovided with ultrasonic energy, which may be within the range of from600 KHz to 2 MHz. This can more improve the efficiency of replacement ofHF with water.

[0078] In the case of the present invention, after the anodization iscompleted, the porous body may preferably be moved from the anodizingsolution to the cleaning solution in a time as short as possible, which,however, is by no means limited to 3 minutes, and may be elongated up toabout 10 minutes.

[0079] A cleaning unit (an anodization-and-cleaning unit) used in thepresent invention will be described below with reference to FIG. 6.

[0080] The cleaning unit shown in FIG. 6 is so constructed as to alsoserve as an anodization unit. This unit is so constructed as to consistchiefly of a bath 61 that functions also as a substrate holder and canhold the anodizing solution and the cleaning solution, a flat-platenegative electrode 63 provided with many holes, and a positive electrode64 up and down movable relatively to the bath 61. The bath 61 is made ofa fluorocarbon type fluorine-resistant material such astetrafluoroethylene resin, and has an opening 69 at the bottom of thebath 61. Then, along the inside edge of this opening 69, a sealingmember 65 such as a substrate suction ring is set to the bottom of thebath 61. The substrate attraction (suction) ring 65 is flat at itsattraction portion, and in its plane a vacuum groove (not shown) isformed which communicates with a communicating path 22 forvacuumizing/pressurizing its space to vacuum-attract orpressurizing-release a treatment target W. The treatment target W, suchas a silicone wafer, is attracted and held to the substrate suction ring65 at its under-side perimeter. In this state, an electrolytic solutionis poured as an anodizing solution 67 into the bath 61 from a feednozzle 27 until the negative electrode 63 stands immersed in theelectrolytic solution. The negative electrode 63 is made of a platinumplate having substantially the same diameter as the treatment target,and a plurality of holes like those made by punching are made in theplate so that reaction by-product gases such as hydrogen generatedduring the anodization can be removed. The positive electrode 64 passesthrough the opening 69 of the bath 61 to come into direct contact withthe back of the treatment target W. This positive electrode 64 by nomeans comes into direct contact with the electrolytic solution, andhence is formed of aluminum. The positive electrode 64 is placed on astand 25 together with an up-and-down means 24, and is provided with acommunicating path 23 for vacuumizing/pressurizing its space tovacuum-attract or pressurizing-release the treatment target W.

[0081] To carry out anodization, the positive electrode 64 is keptraised to the uppermost position by means of the up-and-down means 24,and the treatment target W is placed thereon. The communicating path 23is vaccumized to attract the treatment target W to the positiveelectrode 64 by suction. The positive electrode 64 is descended to bringthe treatment target W into contact with the bottom of the bath 61. Thenthe communicating path 22 is vacuumized to attract the treatment targetW, at its perimeter, to the sealing member 6 by suction. Afteranodization is ready, the aqueous HF solution is fed into the bath 61through the feed nozzle 27. After it has reached a predeterminedquantity, a direct-current voltage is applied across the negativeelectrode 63 and the positive electrode 64. At this time, theanodization may be continued while feeding the aqueous HF solutioncontinuously and allowing it to overflow. In FIG. 6, reference numeral26 denotes an overflow bath.

[0082] After the anodization is completed, the positive electrode 64 isdescended and also the negative electrode 63 is brought aside out of thebath 61. A drain 21 is opened, and the anodizing solution, 67, isdischarged therethrough to once make the bath 61 empty. Thereafter, thecleaning solution containing an alcohol is fed through the feed nozzle.At this time, too, the cleaning may be carried out while allowing thecleaning solution to overflow. The cleaning solution feed nozzle may beprovided independently from the HF solution feed nozzle.

[0083] Subsequently, the drain 21 is again opened to discharge thecleaning solution containing an alcohol, and thereafter the pure wateris fed into the bath 61 through the feed nozzle 27. It is preferable tooptionally attach an ultrasonic vibrator to the feed nozzle 27 or bath61 so that the pure water cleaning solution can be provided withultrasonic energy.

[0084] After the cleaning with pure water, the drain 21 is opened todischarge the pure water. The vacuum attraction through thecommunicating path 22 is stopped, and the positive electrode 64 isascended, where the vacuum attraction through the communicating path 23is stopped to take out the treatment target W.

[0085]FIG. 6 illustrates a state where the positive electrode 64 is atthe descended position, so as to make it easy to understand theconstruction of the unit. During the anodization, it is at the positioncoming into contact with the back of the treatment target W.

[0086]FIG. 7 shows a porous-body cleaning system as another example usedin the present invention.

[0087] The system shown in FIG. 7 has an anodization unit 40 havingsubstantially the same construction as the one shown in FIG. 6, analcohol cleaning unit 41 and a pure-water cleaning unit 42.

[0088] After the anodization is carried out in the anodization unit 40,the treatment target W having been anodized is, after the anodizingsolution in the bath 61 is completely discharged, moved to the alcoholcleaning unit 41 by means of a horizontal transfer robot (not shown). Inan alcohol cleaning bath 33, a rotary chuck holder 34 vacuum-attractsthe treatment target W on its back to hold it. In that state, thecleaning solution containing an alcohol is fed through a nozzle 35provided above the treatment target W. In this unit, the cleaningsolution containing an alcohol is jetted out toward the treatment targetW while rotating it at a predetermined number of revolutions. Thus, thecleaning with an alcohol is effected.

[0089] Next, the treatment target W on which the cleaning with analcohol has been completed is moved to the pure-water cleaning unit 42by means of a horizontal transfer robot. The treatment target W is soarranged as to be held at its peripheral edge with a substrate chuck 37,tightly by an external force. Then, the pure water is fed through anupper nozzle 38 and a lower nozzle 39. In this unit, too, the pure wateris jetted out of both the upper nozzle 38 and the lower nozzle 39 whilerotating the treatment target W at a predetermined number ofrevolutions. Stopping the jetting of pure water, the treatment target Wmay further be spin-dried while continuing its rotation.

[0090]FIG. 8 shows a pure-water cleaning unit used in the presentinvention. This unit is a modification of the pure-water cleaning unit42 shown in FIG. 7, and is a unit having a bath 36 provided with ahermetically closable cover 51 so that, after rinsing with pure water,the hermetically closed space defined by the bath 36 and the cover 51can be evacuated through an exhaust path 52 to carry out spin-dryingwhile keeping vacuum the inside of the hermetically closed space. Wherewater content remains in the pores, it may adversely affect thesubsequent processing steps. For example, when epitaxial growth iseffected on the porous silicon, a trace quantity of water may evaporatefrom the interior of pores of the porous body and diffuse into anepitaxial growth chamber to bring about defects in an epitaxially grownfilm. In order to prevent this, it is fairly effective to carry out suchvacuum deaeration finally.

[0091]FIG. 9 shows an alcohol cleaning unit used in the presentinvention. A cleaning bath 53 is provided therein with a nozzle 54through which the vapor of the cleaning solution containing an alcoholis fed. A treatment target W is inserted from above the bath 53 into it,and the vapor of the cleaning solution containing an alcohol is fedthrough the nozzle 54 to fill the inside of the bath with vapor to carryout cleaning. Thereafter, the treatment target W may slowly be taken outof the bath to the external dry atmosphere, so that the alcoholremaining in the treatment target W vaporizes, thus the treatment targetW can be dried. If the pure water is directly jetted onto the porousbody having a very high porosity, e.g., a porosity of 70% or higher, orultrasonic water is imparted thereto to carry out physical cleaning, theporous body tends to crush. Accordingly, such a unit may be used,whereby even a porous body having a high porosity can be cleaned withoutbreak of the porous body. Porous silicon having a porosity of 70% orhigher is preferably applied as a light-emitting material. Compared withthe conventional case where treatment targets are cleaned with only purewater after anodization, those subjected to cleaning with alcohol vaporand drying after anodization may cause no change in the porousstructure. Hence, the material can maintain light-emission intensitystably for a long time.

[0092] In the foregoing, described is a case where a solution containingan alcohol is used as the cleaning solution. In the present invention,however, acetic acid may also be used in place of the alcohol.

[0093] A process for producing a non-porous film and a process forproducing a bonded substrate according to the present invention will bedescribed below.

[0094]FIG. 10 is a flow chart of a process for producing a bondedsubstrate according to the present invention.

[0095] First, in a step S11, a non-porous body 1 is treated byanodization to form a porous layer at least on its surface.

[0096] In a step S12, the porous layer 2 is cleaned by immersing it in a100% alcohol or an aqueous alcohol solution, by adding a 100% alcohol oran aqueous alcohol solution dropwise to the porous layer 2, or byexposing the porous layer 2 to the vapor of a 100% alcohol or an aqueousalcohol solution. In this step S12, acetic acid may also be used inplace of the alcohol. An example of the cleaning method is as describedabove.

[0097] Next, optionally in a step S13, the porous layer 2 is cleanedwith pure water. In this step, too, the porous layer 2 may be immersedin the pure water, the pure water may be added dropwise to the porouslayer 2, or the porous layer 2 may be exposed to water vapor (steam).Here, it is also preferable to clean it with pure water provided withultrasonic energy by the use of an ultrasonic vibrator. Then, the porouslayer 2 is dried to complete a series of cleaning steps. Examples of thewashing with pure water and drying are also as described above.

[0098] Next, the porous layer 2 is oxidized at a low temperature to forma thin oxide layer on inner wall surfaces of pores.

[0099] Subsequently, in a step S14, a non-porous layer 3 is formed onthe porous layer 2. The step S14 may be effected prior to the step S11to form the non-porous layer 3 on the non-porous body 1 and thereaftermake the whole single-crystal silicon porous from the back.Alternatively, anodization may be so carried out that the back side ofthe non-porous body 1 is made porous to leave the non-porous layer 3 onthe surface side.

[0100] Next, in a step S15, an insulating layer 4 is optionally formedon the surface of the non-porous layer 3 and, as shown in a step S16,the non-porous layer 3 is bonded to a supporting base 5 preparedseparately, via the insulating layer 4 between them.

[0101] In a step S17, pretreatment is made in order to remove the porouslayer 2 from the multi-layer structure thus formed. In the case wherethe non-porous body 1 remains as shown in FIG. 10, this is removed fromthe multi-layer structure by grinding, lapping, polishing or etching.Thereafter, the porous layer 2 uncovered is selectively removed as shownin a step S18, by wet etching with an etchant containing HF, H₂O₂ andwater.

[0102] Alternatively, the non-porous body 1 may be separated (step S17)by applying an external force to the multi-layer structure, or byproducing an internal stress therein, so as to cause a break in theporous layer 2 or at its interface with the upper or lower layer. Someporous layer 2 remaining on the non-porous layer 3 may selectively beremoved (step S18) by etching in the same manner as the above.

[0103] The bonded substrate thus obtained may further optionally besubjected to heat treatment in a reducing atmosphere containinghydrogen, to make its surface more smooth.

[0104] Since the porous layer can almost be free from any change inporous structure due to the HF or aqueous HF solution having remainedtherein, the bonded substrate thus obtained does not cause anymicroscopically uneven film thickness, thus a bonded substrate having ahigh quality level can be obtained.

[0105] The porous layer 2 used in the present invention may includelayers of, as stated previously, semiconductors such as Si, Ge, GaAs,GaAlAs, SiC, SiGe and C. In particular, porous silicon having a porositylower than 70% is preferred as a base material used for epitaxialgrowth. More preferably, the porous layer at its part adjoining oradjacent to the non-porous layer may have a porosity not higher than30%. The porous layer may preferably have a thickness of from about 1 μmto about 30 μm.

[0106] The anodization used in the present invention is carried out inan aqueous HF solution, or an aqueous solution of HF and an alcohol.

[0107] As the alcohol used in the cleaning step of the presentinvention, usable are methanol, ethanol and propanol. The cleaningsolution may be any those containing at least 4% by weight of thealcohol.

[0108] After the cleaning with the cleaning solution containing analcohol, the porous body may preferably be cleaned with pure water.Here, as stated previously it may be cleaned with pure water providedwith ultrasonic energy ranging from 600 KHz to 2 MHz. This can moreimprove the efficiency of replacement of HF with water.

[0109] In the case of the present invention, after the anodization iscompleted, the porous body may preferably be moved from the anodizingsolution to the cleaning solution in a time as short as possible, which,however, is by no means limited to 3 minutes, and may be elongated up toabout 10 minutes.

[0110] The non-porous layer used in the present invention may preferablyinclude, e.g., layers of elemental semiconductors such as Si and Ge,compound semiconductors such as GaAs, GaAlAs, SiC and SiGe, metals, andsuperconductors. Stated specifically, single-crystal silicon layers,polycrystalline silicon layers and amorphous silicon layers arepreferred. In the non-porous layer, device or semiconductor junctionsuch as MOSFET, p-n junction, p-i-n junction and MIS junction may alsobe formed.

[0111] As the insulating layer provided optionally, preferably usableare layers of insulators or dielectrics, such as silicon oxide, siliconnitride and silicon oxide nitride. This layer may be formed as a singlelayer or a plurality of layers formed of like materials or differentmaterials.

[0112] The supporting base used in the present invention may includesemiconductors such as silicon, metals such as aluminum or stainlesssteel, ceramics such as alumina, and insulating materials such as quartzglass and plastic films. These supporting bases may also be those on thesurface of which a layer of a material different from the materialconstituting the supporting base itself has been formed. In the casewhere bonded SOI substrates are produced, the insulating layer maypreferably be formed on the surface of the non-porous layer andthereafter bonded to a silicon wafer or a quartz wafer. Also, thesupporting base may be a jig used only for the separation.

[0113] In order to remove the porous layer selectively, an etchant isused which may achieve an etching rate to porous bodies that is at least10,000 times, and preferably at least 100,000 times, the etching rate tonon-porous bodies. In the case of porous silicon and non-porous silicon,preferably usable are solutions containing HF and an oxidant, asexemplified by a mixture solution of hydrofluoric acid, nitric acid andacetic acid, a mixture solution of hydrofluoric acid, hydrogen peroxidewater and water, a mixture solution of hydrofluoric acid, an alcohol andwater, and a mixture solution of hydrofluoric acid, hydrogen peroxidewater and alcohol.

[0114] (Anodization and Cleaning Units)

[0115] In the present invention, the units shown in FIGS. 6 to 9 may beused as the anodization and cleaning unit. Besides these, anodizationand cleaning units shown in FIGS. 11 to 14 may also be used.

[0116]FIG. 11 shows a holder and a substrate transport robot which areused in an anodization unit in the present invention. FIG. 12 shows theanodization unit. FIGS. 13 and 14 show anodization and cleaning systemsused in the present invention.

[0117] The unit shown in FIG. 12 is a unit by which three substrates astreatment targets can be anodized in one time. As shown in FIG. 11, asubstrate-holding member (hereinafter “holder”) 102 in the anodizationunit is a square plate provide with a circular opening 103 atsubstantially the center thereof. Along the opening 103, a ring-typesubstrate attraction (suction) pad (hereinafter “pad”) 104 is embedded.A groove is made in the pad on its surface, and the interior of thegroove can be brought into a vacuum through an evacuation line 105 fromthe back of the pad. Reference numerals 106 a and 106 b each denote asubstrate transport robot, which operates in pair. First, the robot 106a holds a treatment target W on its back by vacuum attraction and bringsit to come near to the holder 102 so as to be in parallel to its face.Next, the robot 106 b is passed from the opening 103 of the holder 102through the part turning in L-shape, and waits for the treatment targetW to come near. The robot 106 b has the function of vacuum attractionlike the robot 106 a. At the time the back of the treatment target W hascome into contact with the leading end of the robot 106 b, the robot 106b holds the treatment target W by attraction (suction), and the robot106 a release its attraction to go away upward. Subsequently, the robot106 b moves toward the right as viewed in the drawing, so that the backof the treatment target W comes into contact with the pad 104. Here, theinterior of the groove in the pad 104 is kept vacuum by the evacuationline 105, and the pad 104 holds the treatment target W by attraction.The robot 106 b passes through the opening 103 and go away upward. Thus,the treatment target W is held by holder 102. Also, when the treatmenttarget W is detached therefrom, the above procedure is followed inreverse.

[0118] As shown in FIG. 12, an anodizing bath 210 of the anodizationunit is fitted with a negative electrode 206 a and a positive electrode206 b at its both ends. Three holders 102 are arranged in series in sucha way that they are held between these electrodes fitted in pair. FIG.12 shows a state where the treatment target W has already been held byeach holder 102. The spaces between the electrodes 206 a and 206 b andthe holders 102 and between the holders 102 are each filled with anelectrolytic solution 209, and are respectively separated with thetreatment targets. Anodization is carried out in this state by applyinga direct-current voltage across the electrodes 206 a and 206 b. Afterthe anodization is completed, waste-liquid outlets 208 are opened, andthe electrolytic solution 209 is discharge therethrough. These holders102 functions like the bottom of the bath 61 of the unit shown in FIG.6.

[0119] As shown in FIG. 13, an anodization system installed with theabove anodization unit has a loader 301, an anodizing bath 302, acleaning bath 303, a spin dryer 304 and an unloader 305 in this orderfrom the left as viewed in the drawing. In the direction of thisarrangement, a substrate (wafer) sheet-by-sheet transport robot 306 anda carrier transport robot 307 are provided. The substrate sheet-by-sheettransport robot 306 is further constituted of two portions 306 a and 306b as shown in FIG. 13. The cleaning bath 303 is provided with thefunction to circulate an aqueous solution containing at least oneselected from an alcohol and acetic acid and the function to feed purewater. This system also has a system 308 in which the electrolyticsolution in the anodizing bath is circulatingly filtered.

[0120] Substrates as treatment targets W placed in the loader 301 areset in the holders 102 by the wafer transport robot 306, and aredisposed in the anodizing bath 302.

[0121] Substrates having been subjected to anodization in the anodizingbath 302 are taken out of the holders 102 by the robot 306, and aretransported to the cleaning bath 303. In the cleaning bath 303, thesubstrates are cleaned with an aqueous solution containing at least oneselected from an alcohol and acetic acid. Subsequently, they are cleanedwith pure water in the same bath.

[0122] The substrates thus cleaned by the aid of the robot 307 aretransported to a robot dryer 309 together with the carrier, and are oncedried there.

[0123] Subsequently, the substrates are sheet by sheet taken out fromthe carrier, and then spin-dried in the spin dryer 304.

[0124] The substrates having been dried are sent out to the unloader305.

[0125] Thus, the treatment of from the step S11 to the step S13 iscarried out in one lot.

[0126]FIG. 14 shows a modification of the system shown in FIG. 13, inwhich another cleaning bath is added to the cleaning bath providedsolely in the system shown in FIG. 13. A first cleaning bath 310 is abath having the function to circulate the aqueous solution containing analcohol and/or acetic acid, and has a filtration system. In a secondcleaning bath 303, only pure water is supplied and the final cleaningwith water is carried out.

[0127] In the systems of the present invention as shown in FIGS. 13 and14, the units constructed like those shown in FIGS. 6 to 9, 11 and 12may be used as the anodizing bath or the cleaning bath. In FIG. 14,other reference numerals denotes the same as those shown in FIG. 13.

[0128] (Embodiment 1)

[0129] Again with reference to FIG. 1 and others, the process forproducing the non-porous film and bonded substrate according to apreferred embodiment of the present invention will be described ingreater detail by taking the case of silicon as an example.

[0130] Single-crystal silicon wafers are prepared as the non-porous body1 (FIG. 10), the surface of each wafer is made porous in a depth of fromabout 1 μm to about 30 μm by means of the anodizing unit as shown inFIG. 6 or 12, thus a porous single-crystal silicon layer is formed asthe porous layer 2. The porous layer formed here may preferably be madeto have a porosity of from about 5% to 70%, and more preferably from 10%to 50%, in approximation. Also, it is preferable to change anodizingcurrent density, HF concentration and so forth in the course of theanodization so that the porous layer can be made to have an at leasttwo-layer multi-layer structure having porosities different from oneanother.

[0131] Subsequently, using the system shown in FIGS. 7 to 9 and 13 or14, the silicon wafers whose surfaces have been made porous are cleanedwith a cleaning solution comprised of an aqueous solution containing analcohol and/or acetic acid in a concentration of at least 4% by weight.Thereafter, the cleaning solution is replaced with pure water to cleanthe silicon wafers, followed by drying.

[0132] The silicon wafers thus cleaned are each optionally subjected toheat treatment at about 200° C. to about 600° C. to oxidize the innerwalls of pores in the porous layer to form oxide films on the inner-wallsurfaces. The pore walls remain consisting chiefly of silicon.

[0133] On the porous layer 2, the non-porous layer 3 comprised ofsingle-crystal silicon is formed by CVD, sputtering, molecular-beamepitaxy or liquid-phase epitaxy.

[0134] On the surface of the non-porous layer 3, a silicon oxide film isoptionally formed as the insulating film 4.

[0135] The surface of the insulating film 4 and the surface of asingle-crystal silicon wafer or supporting base 5 comprised of quartzglass are brought into contact to bond them. In the case where theinsulating film 4 is not formed, the non-porous layer 3 is bonded to thesupporting base 5. In order to enhance their bond strength, themulti-layer structure thus formed by bonding may be subjected to heattreatment in an atmosphere of an inert gas or in an atmosphere of anoxidative gas, or to anodic bonding.

[0136] From the multi-layer structure, the silicon wafer 1 havingremained without being made porous is removed by grinding, lapping orRIE (reactive ion etching) on the back side, which is opposite to thebonded-face side.

[0137] The porous layer 2 thus uncovered is further selectively etchedwith the etchant as described previously. Thus, a bonded substratehaving a non-porous film on the supporting base 5 is obtained.

[0138] The treatment to make the silicon substrate porous byanodization, i.e., to form pores therein is carried out in, e.g., theaqueous HF solution. For this treatment, the presence of holes insilicon crystal is known to be indispensable, and the mechanism of theirreaction is presumed as follows.

[0139] First, holes in the silicon substrate having undergoneelectrolysis in the aqueous HF solution are induced to the surface ofthe negative-electrode side. As a result, the density of Si—H bondsincreases which are present in such a form that they compensate unbondedarms at the surface. At this time, fluorine ions in the aqueous HFsolution attack the Si—H bonds nucleophilically to form Si—F bonds. As aresult of this reaction, one electron is released to thepositive-electrode side simultaneously with the generation of H₂molecules. Because of the polarization characteristics of the Si—Fbonds, Si—Si bonds in the vicinity of the surface become weak. Such weakSi—Si bonds are attacked by HF or H₂O, and the Si atoms on the crystalsurface turn into SiF₄ to become liberated from the crystal surface. Asa result, hollows (concavities) are produced on the crystal surface, anddistribution of electric fields that attracts holes preferentiallyoccurs at this part, so that such surface heterogeniety expands and theetching of silicon atoms progresses continuously along the electricfields. Incidentally, the solution used in the anodization is notlimited to the aqueous HF solution, and may be other electrolyticsolutions.

[0140] The etching of silicon in the step of anodization is explained bythe following reaction scheme.

[0141] Si+2HF+(2−n)e⁺→SiF₂+2H⁺+ne⁻

[0142] 2SiF₂→Si+SiF₄

[0143] SiF₄+2HF→H₂SiF₆

[0144] Namely, the reaction with hydrofluoric acid brings about theformation of a silicone compound H₂SiF₆, so that the silicon is etched.The above reaction scheme also shows that the H₂SiF₆ is formed in alarger quantity with an increase in the concentration of HF. This H₂SiF₆has properties that it is very hardly reactive with, i.e., sparinglysoluble in, acid (including hydrofluoric acid) or alkali solutions.

[0145] The progress of anodization reaction also brings about theformation of pores of tens to hundreds of angstroms at the substratesurface, and the pores extend ahead along the direction of electricfields. Namely, the electrolytic solution (aqueous hydrofluoric acidsolution) enters the pores and causes the reaction at the tip of thepores. Then, even when the electric fields disappear, the pores in whichthe hydrofluoric acid solution is kept confined generate at random. Whatcomes into question at this stage is that the hydrofluoric acid solutionconfined in the pores continues the reaction even after the electricfields have disappeared, to continue forming the H₂SiF₆. Where theH₂SiF₆ thus formed adheres to the inner walls of the pores, theselective etching of the porous silicon layer, carried out finally inthe process, becomes uneven.

[0146] In the selective etching of porous layers, the non-porous layerto be left without being etched and the porous layer to be removed areboth the alike single-crystal silicon in many cases. Hence, althoughchemical etching rate should be equal in principle, the etchant havingsoaked on into the pores of the porous layer etches the pore wallsurfaces, so that the porous layer comes to be etched not only from thelayer surface but also from the interior. Thus, the etching of theporous body stands in a mode where the whole layer breaks downphysically.

[0147] Accordingly, in order to etch the porous layer uniformly, theH₂SiF₆ must be kept from its random generation in the pores. For thisend, it is important to replace the HF which remains easily in thepores, with a liquid having no etching action as far as possible. Also,in order to remove the porous layer uniformly, it is preferable to formoxide films on the pore inner walls of the porous layer, where it isalso desirable to keep the H₂SiF₆ from being formed.

[0148]FIGS. 15A to 15D diagrammatically show how the interior of poresin the porous layer stands when cleaned with pure water and dried afterthe anodization is completed.

[0149]FIG. 15A shows a cross section of the porous layer immediatelyafter it has been taken out to the atmosphere after the anodization iscompleted. Pores 602 are formed as a result of the anodization of asubstrate 601, and an electrolytic solution 80 remains in the pores. Theelectrolytic solution is, as stated previously, a mixture solution of HFand an alcohol in many cases.

[0150]FIG. 15B shows how the interior of pores stands after the porouslayer has been left for few minutes in the atmosphere. The water contentor alcohol content in the electrolytic solution 80 has evaporated inpart and the aqueous HF solution stands remaining in the depths of thepores in the state it has been concentrated.

[0151]FIG. 15C shows how the porous layer is cleaned with pure water 82.

[0152] In general, liquid enters the pores by capillarity, and is mixedwith thick aqueous HF solution. Subsequently the hydrofluoric aciddiffuses on toward the outside of the pores to become replaced graduallywith the pure water in the pores. Thus, the cleaning is effected.

[0153] At this stage, the entering depth H in the case where the liquidenters the pores by capillarity is expressed by the following equation.

H=2γ·cosθ/aρg

[0154] where γ is surface tension; θ, contact angle of the liquid withrespect to the substrate; a, pore size of the porous body; ρ, density ofthe liquid; and g, acceleration of gravity. At this stage, the surfaceof the porous layer stands hydrophobic on account of the hydrofluoricacid, and hence has a very large contact angle θ. Hence, the enteringdepth H of the water is nearly zero. Namely, the pure water 82 forcleaning can little enter the pores 602. For this reason, a layer 604 ofair is necessarily formed in the vicinity of the surfaces of the pores602. Once this has occurred, even when the water is tried being removedby drying with a spin dryer or the like after the cleaning with purewater, the hydrofluoric acid in the pores 602 is not replaced with thewater to become higher and higher in concentration. Finally the solutionremaining in the pores 602 dries up completely, so that, as shown inFIG. 15D, a secondary-product dry matter 606 adheres to the pore wallsurfaces. This dry matter 606 is the H₂SiF₆ stated above. In FIG. 15D, astate is illustrated where the dry matter 606 stands adhered to all thepores. Actually, it adheres to only part of the massive pores, or drymatters 606 having different thickness for each pore may occur, tobecome uneven.

[0155] On the other hand, FIGS. 16A to 16C diagrammatically show how theinterior of pores in the porous layer stands when cleaned with theaqueous solution containing an alcohol and/or acetic acid, after theanodization is completed.

[0156]FIG. 16A shows a cross section of the porous layer immediatelyafter it has been taken out to the atmosphere after the anodization iscompleted. Reference numeral 701 denotes a substrate; 702, pores; and80, an electrolytic solution.

[0157]FIG. 16B shows how the porous layer is cleaned with the aqueoussolution containing an alcohol and/or acetic acid. On account of thealcohol and/or acetic acid that acts like a surface-active agent, theabove contact angle θ is so small that the solution can enter the poreswith ease. Hence, the aqueous solution containing an alcohol and/oracetic acid, 83, and the electrolytic solution 80 intermingle fast witheach other. Then, the cleaning with water may sufficiently be carriedout, so that the concentration of the electrolytic solution 80 is welllowered and almost all the electrolytic solution in the pores isreplaced with water. This is then dried with a spin dryer or the like,thus, as shown in FIG. 16C, a porous silicon layer where any secondaryproducts ascribable to HF are present in the pores can be obtained.

[0158] (Embodiment 2)

[0159] The process for producing the non-porous film and bondedsubstrate according to another preferred embodiment of the presentinvention will be described in greater detail by taking the case ofsilicon as an example.

[0160] Single-crystal silicon wafers are prepared as the non-porous body1, the surface of each wafer is made porous in a depth of from about 1μm to about 30 μm by means of the anodizing unit as shown in FIG. 6 or12, thus a porous single-crystal silicon layer is formed as the porouslayer 2. The porous layer formed here may preferably be made to have aporosity of from about 5% to 70%, and more preferably from 10% to 50%,in approximation. Also, anodizing current density, HF concentration orthe like is changed in the course of the anodization so that the porouslayer can be made to have an at least two-layer multi-layer structurehaving porosities which are higher inside the substrate than at thesurface.

[0161] Subsequently, using the system shown in FIGS. 7 to 9 and 13 or14, the silicon wafers whose surfaces have been made porous are cleanedwith a cleaning solution comprised of an aqueous solution containing analcohol and/or acetic acid in a concentration of at least 4% by weight.Thereafter, the cleaning solution is replaced with pure water to cleanthe silicon wafers, followed by drying.

[0162] The silicon wafers thus cleaned are each optionally subjected toheat treatment at about 200° C. to about 600° C. to oxidize the innerwalls of pores in the porous layer to form oxide films on the inner-wallsurfaces.

[0163] On the porous layer 2, the non-porous layer 3 comprised ofsingle-crystal silicon is formed by CVD, sputtering, molecular-beamepitaxy or liquid-phase epitaxy.

[0164] On the surface of the non-porous layer 3, a silicon oxide film isoptionally formed as the insulating film 4.

[0165] The surface of the insulating film 4 and the surface of asingle-crystal silicon wafer or supporting base 5 comprised of quartzglass are brought into contact to bond them. In order to enhance theirbond strength, the resultant multi-layer structure may be subjected toheat treatment in an atmosphere of an inert gas or in an atmosphere ofan oxidative gas, or to anodic bonding.

[0166] Then, a dividing member such as a wedge or a blade is inserted tothe side face of the multi-layer structure to divide the multi-layerstructure into two parts. Thus, the porous layer, having a lowmechanical strength, is cracked at its interior or interface and themulti-layer structure is divided into two parts. A fluid such as liquidand gas may be jetted to the side face of the multi-layer structure todivide the multi-layer structure mechanically. Alternatively, themulti-layer structure may be irradiated by light to generate heattherein or may be heated from the outside so that an internal stress canbe produced in the multi-layer structure, and the force thereby producedmay be utilized to divide the multi-layer structure.

[0167] As a result of this dividing, the non-porous layer comes to havebeen transferred onto the supporting base. Since a residual layer of theporous layer 2 is present on this non-porous layer, this layer isselectively etched with the etchant described previously. According tothe present embodiment, any unwanted secondary product does not remainin the pores of the porous layer, and hence the non-porous layer afterthe etching can be free from any uneven film thickness.

[0168] Then, the non-porous layer on the supporting base may optionallybe subjected to heat treatment in a reducing atmosphere containinghydrogen, to make the surface smooth and also outwards diffuse andremove boron and so forth contained in the non-porous layer.

[0169] Thus, a bonded substrate having the non-porous layer can beobtained which is preferable as an SOI substrate.

[0170] (Embodiment 3)

[0171] The process for producing the non-porous film and bondedsubstrate according to still another preferred embodiment of the presentinvention will be described in detail by taking the case of silicon asan example.

[0172] Single-crystal silicon wafers are prepared as non-poroustreatment targets. These are each subjected to anodization using theanodizing unit shown in FIG. 6 or 12, to form a porous silicon layer 2as shown in the step S11 in FIG. 10.

[0173] As conditions for the anodization, first, using a mixturesolution of hydrofluoric acid and ethanol, a low-porosity, first porouslayer having a porosity of from 5% to 30% and a thickness of from 1 μmto 29 μm may be formed under a low current density. Then, after thechanging to a high current density, a high-porosity, second porous layerhaving a porosity of from 30% to 70% and a thickness of from 1 nm to 3μm may be formed beneath the first porous layer.

[0174] Next, as shown in the step S12 in FIG. 10, the cleaning iscarried out using the system shown in FIGS. 7 to 9 and 13 or 14. Thus,the interior of pores of the porous silicon layer 2 having thedouble-layer structure with porosities different from each other iscleaned with the cleaning solution containing an alcohol and/or aceticacid. In the case where the low-porosity layer is formed on the surfaceside, the cleaning of the present invention is particularly effective.

[0175] Thereafter, as shown in the step S13 in FIG. 10, the poroussilicon layer 2 is cleaned with pure water, followed by drying.

[0176] The porous silicon layer 2 is subjected to heat treatment at 200°C. to 600° C. in an oxidizing atmosphere to form oxide films on the poreinner walls.

[0177] Oxide film formed on the layer surface of the porous siliconlayer 2 whose pore inner walls have been oxidized is removed withdiluted hydrofluoric acid. At this stage, the greater part of the oxidefilms on the pore inner walls stand remaining.

[0178] The wafers on each surface of which the porous silicon layer 2has been formed are set in an epitaxial growth apparatus, and the poroussilicon layer 2 is prebaked in an atmosphere of hydrogen at atemperature raised to 900° C. to 1,000° C.

[0179] A silicon gas such as SiH₄ is introduced to stop up the surfacepores of the porous silicon layer 2. The introducing of silicon gas isonce stopped, and heat treatment is made in an atmosphere of hydrogen ata temperature raised to 1,000° C. to 1,200° C. Then a silicon gas suchas SiH₂Cl₂ is introduced to carry out epitaxial growth at a temperaturedropped to 900° C. to 1,000° C., to form a non-porous layer 3 on theporous silicon layer 2 whose pore inner walls have been oxidized.

[0180] An insulating layer is formed on the non-porous layer 3.

[0181] The insulating layer formed is bonded to the supporting base 5 toform a multi-layer structure, followed by heat treatment at 1,000° C. to1,200° C.

[0182] A wedge made of resin or metal is inserted to the side face ofthe multi-layer structure to cause the porous layer of the multi-layerstructure to be cracked at its side face.

[0183] A fluid comprised of liquid or gas is jetted to the side face ofthe multi-layer structure to cause the porous layer to be further inwardcracked.

[0184] Thus, the multi-layer structure is divided at the interfacebetween the high-porosity second porous layer and the low-porosity firstporous layer.

[0185] The low-porosity first porous layer having oxide films in itspore inner walls and remaining on the non-porous layer 3 transferred tothe supporting base 5 side is removed by etching or the like. Since theHF in the pores has well been removed by cleaning after the anodization,the remaining porous layer can uniformly be removed.

[0186] The surface of the non-porous layer 3 is made smooth by hydrogenannealing or the like.

[0187] The non-porous layer 3 thus obtained can have a smooth surfacefree of any uneven film thickness.

EXAMPLES Example 1

[0188] A silicon wafer as a treatment target was set in the unit shownin FIG. 6, and as an electrolytic solution an anodizing electrolyticsolution prepared by mixing hydrofluoric acid of 49% by weight in HFconcentration, water and ethanol in a volume ratio of 1:1:1 was fed intothe bath. A direct-current voltage providing a constant electric currentof 7 mA/cm² was applied to the wafer for 10 minutes. As a result, aporous silicon layer having a porosity of about 20% was uniformly formedon the one surface of the wafer in a thickness of 12 μm.

[0189] Subsequently, the electrolytic solution was discharged throughthe drain, and a cleaning solution prepared by mixing isopropanol andwater in a volume ratio of 1:1 was poured into the bath from the upperpart. This cleaning solution was retained for 1 minute in the state thebath is filled with it, to carry out cleaning, and thereafter thecleaning solution was discharged outside the bath.

[0190] Next, pure water was poured from the upper part. The pure waterwas allowed to overflow to clean (rinse) the wafer for about 20 minutes.Then the wafer was taken out, and dried by means of a spin dryer.Thereafter, this wafer was left in the atmosphere for a week, but nochange was seen at all in the porous silicon of the wafer.

Comparative Example 1

[0191] In Example 1, the cleaning step making use of the cleaningsolution containing an alcohol as described above was omitted, and thewafer cleaned with pure water was spin-dried immediately after theanodization. The wafer obtained was left in the atmosphere for 10 hours.As a result, the porous silicon surface of the wafer changed instructure to become cloudy.

Example 2

[0192] Using the unit 40 shown in FIG. 7, anodization was carried outunder the same conditions as in Example 1. Next, after the anodizingelectrolytic solution was completely discharged, the wafer thus anodizedwas moved to the cleaning unit 41 by means of the horizontal transferrobot. The wafer was rotated at 500 r.p.m. while feeding ethanol (100%)from the nozzle to the wafer to clean the wafer surface for 20 seconds.

[0193] Subsequently, the wafer was moved to the pure-water cleaning unit42 by means of the horizontal transfer robot, where the wafer wasrotated at 400 r.p.m. while jetting out pure water from the upper nozzleand lower nozzle to clean the wafer for 15 minutes. After the cleaning,the jetting of pure water was stopped, and the wafer was rotated at 800r.p.m. in the same unit to carry out drying.

[0194] The porous silicon surface of the wafer cleaned in this way didnot change even after it was left for a week in the same manner as inExample 1, and maintained a very stable state.

Example 3

[0195] In this example, using the unit shown in FIG. 7, the anodization,the cleaning with alcohol, the cleaning with pure water and the dryingwere carried out in the same manner as in Example 2 except that in thecleaning with pure water an ultrasonic vibrator was built in at the tipof the upper nozzle so that the pure water provided with ultrasonicvibration was fed to the wafer to more improve the effect of cleaning.

[0196] While in Example 2 the cleaning with pure water had to be carriedout for 15 minutes, the same cleaning effect as that in Example 2 wasobtained by the cleaning for 10 minutes because of the use of the purewater made to have ultrasonic action.

Example 4

[0197] Example 4 is an example in which the cleaning with pure water andthe drying were carried out using the unit shown in FIG. 8.

[0198] The anodization and the cleaning with alcohol were carried outunder the same conditions as in Example 2. Thereafter, the substrate wascleaned with pure water using the unit shown in FIG. 8. Subsequently, inthe state the substrate was kept hermetically closed in the same bath,it was spin-dried and simultaneously dried by vacuum deaeration. As aresult, the water content little remained in the pores, and stablerporous silicon was obtainable.

Example 5

[0199] Using as an electrolytic solution an aqueous solution of 10% byvolume of hydrofluoric acid, a voltage was applied for 7 minutes so asto provide a constant electric current of 10 mA/cm² in current densityat the time of anodization, to make porous an n-type silicon waferhaving a resistivity of 0.007 Ω·cm. As a result, a porous silicon layerhaving a porosity of about 70% was uniformly formed on the one surfaceof the wafer in a thickness of 12 μm. This wafer was immediately put inthe cleaning unit shown in FIG. 9, to carry out alcohol cleaning usingpropanol vapor, and the wafer thus cleaned was dried while drawing itupwards.

[0200] Because of the employment of this cleaning and drying method,even the high-porosity porous silicon was cleanable without causing anychange in its structure.

[0201] As described above, in the cleaning of porous bodies, after theanodization the porous body is exposed to the atmosphere of cleaningsolution or cleaning vapor containing an alcohol. This makes it possibleto prevent the deterioration of porous bodies themselves which has everbeen caused because of insufficient removal of the anodizing solutionremaining in pores, to prevent the corrosion of surrounding devices dueto components of evaporated anodizing solutions, and also to prevent thecontamination caused by these.

Example 6

[0202] As a treatment target, a 6-inch p-type (0.01-0.02 Ω·cm) siliconwafer (thickness: 625 μm) was prepared. Using the unit shown in FIG. 6,as an electrolytic solution an aqueous HF solution prepared by mixinghydrofluoric acid of 49% by weight in HF concentration and ethanol in avolume ratio of 2:1 was fed into the anodizing bath. Anodizationelectric current was set at 1 mA/cm², and anodization was continued for11 minutes to form a porous silicon layer on the surface of the siliconwafer. The silicon wafer having been thus anodized was immersed in anaqueous solution containing 10% by weight of isopropanol, and was leftfor 3 minutes. Thereafter, the silicon wafer was immersed in pure waterfor 10 minutes to clean it, followed by drying.

[0203] This silicon wafer was subjected to heat treatment at 400° C. for1 hour in an atmosphere of oxygen in an oxidizing furnace to oxidizepore wall surfaces of the porous layer. Next, the oxide film formed onthe porous layer surface was removed with an aqueous HF solution. Then,the wafer was put in a CVD system to carry out baking in an atmosphereof hydrogen, followed by epitaxial growth to form, on the porous layerwhose pore wall surfaces have been oxidized, a 0.3 μm thick epitaxiallayer formed of non-porous single-crystal silicon. The surface of thisepitaxial layer was oxidized at 1,100° C. by hydrogen combustion to forma 0.2 μm thick silicon oxide film. Subsequently, this was bonded to a6-inch silicon wafer prepared separately, followed by heat treatment at1,100° C. for 2 hours in an atmosphere of nitrogen and oxygen to obtaina multi-layer structure. The back of the silicon wafer whose surface wasmade porous was grounded in a depth of about 615 μm by means of agrinder called a back grinder, to make the porous silicon layeruncovered. The multi-layer structure having the porous silicon layermade uncovered was immersed in a solution prepared by mixinghydrofluoric acid and hydrogen peroxide water in a volume ratio of1:100, to remove the porous silicon layer by selective etching.

[0204] As a result of observation of the bonded substrate thus obtained,any spotlike uneven film thickness was not observable as shown in FIG.17. Thereafter, the bonded substrate was subjected to hydrogen annealingto obtain an SOI substrate having an active layer in a thickness of 0.2μm and a buried oxide film in a thickness of 0.2 μm, which was formed ofnon-porous single-crystal silicon and had a smooth surface.

Example 7

[0205] The same silicon wafer as that in Example 6 was prepared, and wassubjected to anodization under the same conditions as in Example 6.

[0206] After the anodization, the wafer was immersed for 3 minutes in abath filled with a cleaning solution comprised of pure water to which15% by weight of isopropanol was added. Thereafter, the cleaningsolution was discharged out of the bath, and then the same bath wasfilled with pure water to clean the wafer with the pure water for 10minutes.

[0207] Thereafter, the same treatment as that in Example 6 was carriedout to etch the porous silicon layer selectively.

[0208] The above process yielded an SOI substrate having an active layerin a thickness of 0.2 μm and a buried oxide film in a thickness of 0.2μm.

Comparative Example 2

[0209] The same silicon wafer as that in Example 6 was prepared, and wassubjected to anodization under the same conditions as in Example 6. Thewafer having been thus anodized was immersed in pure water, and was leftfor 10 minutes to effect cleaning, followed by drying.

[0210] Next, the same treatment as that in Example 6 was carried out toetch the porous silicon layer selectively.

[0211] As a result of observation of the bonded substrate thus obtained,spotlike uneven film thickness 11 like the one shown in FIG. 21 wasobservable which was 2 mm to 7 mm in diameter and was a film thicknesssmaller by about 2 nm to about 7 nm than that of the surrounding area.This uneven film thickness having thus occurred did not easily disappeareven when the wafer was thereafter subjected to hydrogen annealing.

[0212] As described above, in the cleaning of porous bodies, after theanodization the porous body is exposed to the atmosphere of cleaningsolution or cleaning vapor containing an alcohol. This makes it possibleto prevent the uneven film thickness from occurring which has ever beencaused because of insufficient removal of the anodizing solutionremaining in pores.

What is claimed is:
 1. A method of cleaning a porous body formed byanodization, comprising the step of cleaning the porous body after theanodization is completed, with a cleaning solution containing at leastone of an alcohol and acetic acid.
 2. The method of cleaning a porousbody according to claim 1, wherein after the cleaning step the porousbody is further cleaned with pure water.
 3. The method of cleaning aporous body according to claim 1, wherein the cleaning solution is anaqueous solution containing an alcohol.
 4. The method of cleaning aporous body according to claim 1, wherein the cleaning solutioncomprises an alcohol.
 5. The method of cleaning a porous body accordingto claim 1, wherein the cleaning step comprises the step of cleaningwith a cleaning solution comprising an alcohol and the step of cleaningwith a cleaning solution comprising an aqueous solution of an alcohol.6. The method of cleaning a porous body according to claim 1, whereinthe cleaning step comprises the step of exposing the porous body to avapor of the cleaning solution.
 7. The method of cleaning a porous bodyaccording to claim 1, wherein the cleaning step comprises the step ofimmersing the porous body in the cleaning solution.
 8. The method ofcleaning a porous body according to claim 1, wherein after the cleaningstep the porous body is cleaned with pure water provided with anultrasonic energy.
 9. The method of cleaning a porous body according toclaim 8, wherein the porous body has a region having a porosity lowerthan 70%.
 10. The method of cleaning a porous body according to claim 1,wherein after the cleaning step the porous body is cleaned with purewater and thereafter the porous body thus cleaned is spin-dried and isfurther dried by deaeration.
 11. The method of cleaning a porous bodyaccording to claim 1, wherein the cleaning step comprises exposing theporous body to a vapor of the cleaning solution, and moving the porousbody thus cleaned, relatively from the inside of the vapor to theoutside of the vapor to effect drying.
 12. The method of cleaning aporous body according to claim 11, wherein the porous body has a regionhaving a porosity of 70% or higher.
 13. The method of cleaning a porousbody according to claim 1, wherein the porous body is formed on thesurface of a non-porous body base member.
 14. The method of cleaning aporous body according to claim 1, wherein the porous body comprises asemiconductor.
 15. A process for producing a non-porous film, comprisingthe step of forming a non-porous layer on the porous body having beencleaned by the method of cleaning a porous body according to claim 1.16. The process for producing a non-porous film according to claim 15,which further comprises the step of removing the porous body.
 17. Aprocess for producing a non-porous film, comprising the steps of forminga porous layer by anodization, forming a non-porous layer on the porouslayer, bonding the non-porous layer to a supporting base, and removingthe porous layer, wherein; the process further comprises the step of,after the anodization is completed, cleaning the porous layer with acleaning solution containing at least one of an alcohol and acetic acid.18. The process for producing a non-porous film according to claim 17,wherein after the cleaning step the porous layer is further cleaned withpure water.
 19. The process for producing a non-porous film according toclaim 17, wherein the cleaning solution is an aqueous solutioncontaining an alcohol.
 20. The process for producing a non-porous filmaccording to claim 17, wherein the cleaning solution comprises analcohol.
 21. The process for producing a non-porous film according toclaim 17, wherein the cleaning step comprises the step of cleaning witha cleaning solution comprising an alcohol and the step of cleaning witha cleaning solution comprising an aqueous solution of an alcohol. 22.The process for producing a non-porous film according to claim 17,wherein the cleaning step comprises the step of exposing the porouslayer to a vapor of the cleaning solution.
 23. The process for producinga non-porous film according to claim 17, wherein the cleaning stepcomprises the step of immersing the porous layer in the cleaningsolution.
 24. The process for producing a non-porous film according toclaim 17, wherein after the cleaning step the porous layer is cleanedwith pure water provided with an ultrasonic energy.
 25. The process forproducing a non-porous film according to claim 24, wherein the porouslayer has a region having a porosity lower than 70%.
 26. The process forproducing a non-porous film according to claim 17, wherein after thecleaning step the porous layer is cleaned with pure water and thereafterthe porous layer thus cleaned is spin-dried.
 27. The process forproducing a non-porous film according to claim 17, which furthercomprises the step of oxidizing the porous layer to form oxide films onthe pore wall surfaces of the porous layer.
 28. The process forproducing a non-porous film according to claim 17, wherein the porouslayer comprises a semiconductor.
 29. A process for producing a porousbody, comprising the steps of: subjecting a non-porous body toanodization to form a porous body; and cleaning the porous body afterthe anodization is completed, with a cleaning solution containing atleast one of an alcohol and acetic acid.
 30. The process for producing aporous body according to claim 29, wherein after the cleaning step theporous body is further cleaned with pure water.
 31. The process forproducing a porous body according to claim 29, wherein the cleaningsolution is an aqueous solution containing an alcohol.
 32. The processfor producing a porous body according to claim 29, wherein the cleaningsolution comprises an alcohol.
 33. The process for producing a porousbody according to claim 29, wherein the cleaning step comprise s thestep of cleaning with a cleaning solution and is further dried bydeaeration.
 39. The process for producing a porous body according toclaim 29, wherein the cleaning step comprises exposing the porous bodyto a vapor of the cleaning solution, and moving the porous body thuscleaned, relatively from the inside of the vapor to the outside of thevapor to effect drying.
 40. The process for producing a porous bodyaccording to claim 39, wherein the porous body has a region having aporosity of 70% or higher.
 41. The process for producing a porous bodyaccording to claim 29, wherein the porous body is formed on the surfaceof a non-porous body base member.
 42. The process for producing a porousbody according to claim 29, wherein the porous body comprises asemiconductor.
 43. A process for producing a bonded substrate,comprising the steps of forming a porous layer by anodization, bondingto a supporting base a non-porous layer formed on the porous layer, andremoving the porous layer, where in; the process further comprises thestep of, after the anodization is completed, cleaning the porous layerwith a cleaning solution containing at least one of an alcohol andacetic acid.
 44. The process for producing a bonded substrate accordingto claim 43, wherein after the cleaning step the porous layer is furthercleaned with pure water.
 45. The process for producing a bondedsubstrate according to claim 43, wherein the cleaning solution is anaqueous solution containing an alcohol.
 46. The process for producing abonded substrate according to claim 43, wherein the cleaning solutioncomprises an alcohol.
 47. The process for producing a bonded substrateaccording to claim 43, wherein the cleaning step comprises the step ofcleaning with a cleaning solution comprising an alcohol and the step ofcleaning with a cleaning solution comprising an aqueous solution of analcohol.
 48. The process for producing a bonded substrate according toclaim 43, wherein the cleaning step comprises the step of exposing theporous layer to a vapor of the cleaning solution.
 49. The process forproducing a bonded substrate according to claim 43, wherein the cleaningstep comprises the step of immersing the porous layer in the cleaningsolution.
 50. The process for producing a bonded substrate according toclaim 43, wherein after the cleaning step the porous layer is cleanedwith pure water provided with an ultrasonic energy.
 51. The process forproducing a bonded substrate according to claim 50, wherein the porouslayer has a region having a porosity lower than 70%.
 52. The process forproducing a bonded substrate according to claim 50, wherein after thecleaning step the porous layer is cleaned with pure water and thereafterthe porous layer thus cleaned is spin-dried.
 53. The process forproducing a bonded substrate according to claim 43, which furthercomprises the step of oxidizing the porous layer to form oxide films onthe pore wall surfaces of the porous layer.
 54. The process forproducing a bonded substrate according to claim 43, wherein the porouslayer comprises a semiconductor.